Inspiration

How is it possible to get air inside of our bodies?

If I want to get air inside a room, I would have to do a few things.
First of all, I'd have to have a passageway for the air-- like an open door or
window. Then I would need some force to push or draw the air into the room.
The air could be pushed into the room from outside. For example, a fan located in
the hall outside of the room could be used to push the air into the room through the door,
or the wind outside could push air into the room through the window. The air could
also be drawn into the room from inside. For example, a fan could be positioned in
front of the window inside the room, and draw air in through the window.

Well, in our bodies, we have a clear passageway for the air, from
outside into our nasal cavity (or mouth), to our pharynx, trachea, bronchi, bronchioles,
and eventually into the alveoli. We have to get the air into our alveoli, because
that is where all gas exchange will occur.

The other thing we need is a way to push or draw air into that
passageway. We can't sit in front of a fan all day! We certainly have no way
to push it in (or you'd see something outside of our noses all day), so there has to be a
way to draw it in. You may now be thinking about how something could act like a fan
within our bodies, drawing air into our alveoli... at least I hope that is what you are
thinking. It works something like that.

Think back to the fan example. A fan is a force on the air.
When it forces the air to move, we can talk about the fan increasing the air
pressure on the air it moves. So, what we really need is a way to create air
pressure to draw the air into our bodies. Right?

The air in our world sits at a rather constant pressure, called the atmospheric
pressure, of 760 mm Hg. Note that air pressure, just like blood pressure,
is measured in units of "mm Hg." If we want to draw air into our bodies,
we have to be able to make our respiratory tract have a lower pressure than 760
mm Hg. It has to be lower, because all items tend to move from high pressure toward
low pressure without any extra energy needed.

Therefore, the entire question of how we inhale can be narrowed down
to the more specific question: "how do we decrease the pressure within our
respiratory tract?" Keep in mind that we have to do this over and over, since
breathing is cyclical.

How do we decrease the pressure within our respiratory tract?

To understand this, you may need to picture what air pressure really
is. Air is made up of many molecules. All molecules are in constant, random
motion. As the molecules move around, the amount that they bang against each other
and against any container that they are in is their pressure. I have tried to
demonstrate this for you in the animation below.

At
first, the air molecules are just moving around normally. Notice that their movement
is pretty random (at least I tried to make it random). Then, I have gone ahead and
changed the container within which they are moving. I made the container
larger. Once the container is larger, the molecules, still moving at the same rate,
are now less likely to bump into each other or into the walls of the container. You
should at least be able to see that the air molecules tend to be more spread out when the
container is larger. Less bumping around of air molecules reflects less pressure of
the air.

This is the trick, then. In order to decrease the pressure
within our respiratory tract, we have to expand our container. Our container is
basically our thorax. If we can expand our thorax, the air pressure within our
thoracic cavity will fall, and air will rush into our respiratory tract.

The diaphragm is our primary means to increase our
thoracic cavity (to decrease the air pressure in our thoracic cavity).
The diaphragm lies at the base of
the thorax, separating the thoracic cavity from the abdominal cavity. The diaphragm
is a sheet of skeletal muscle; it can contract under automatic or voluntary control.
At rest, however, this sheet of muscle is not flat; instead, it has a curved shape to
it. When you view the diaphragm from anterior or from the side, you can see that the
diaphragm is curved. Therefore, when the diaphragm contracts, it flattens out.
This flattening out of the diaphragm, the inferior aspect of the thorax, causes the size
of the thorax to increase. I have tried to show this to you in this tiny
animation. Your book also describes this, in Figure 19.22, which I have also
included for you here.

and here are some real pictures of a diaphragm, taken from the NPAC
Visible Human Viewer,
and then animated a teeny bit to point out the diaphragm to you.

Other muscles can help to expand the thoracic cavity

The external intercostal muscles of the thorax are
also often involved in inspiratory breathing. When these muscles contract, they act
to raise the ribs and elevate the sternum. You see, the contraction of the diaphragm
increased the thoracic volume by lowering the bottom border of the thorax, while the
external intercostal muscles increase thoracic volume by raising the top border of the
thorax.

For really deep inspiratory breaths, like before you dive
underwater, the external intercostal muscles contract more, and other muscles can also
participate. The pectoralis minor muscles and the sternocleidomastoid muscles can
pull on the rib cage and sternum more, to raise it and draw it out further.

Please note that the ease with which the lungs can be expanded is
called the compliance of the lungs.

Pleural membranes assist in the expansion of the lungs

There are parietal and visceral membranes around the lungs, just
like you saw for the heart. The lung membranes are called pleural membranes, so
there is a visceral pleura that is firmly attached to the lung tissue
itself, and a parietal pleura that is attached to the thoracic body wall.
Between these membranes, there is a tiny, almost non-existent space, called the pleural
cavity. Since the visceral and parieta pleurae are serous membranes, they
secrete serous fluid. Therefore, serous fluid lies in the pleural cavity.

When the thorax expands, that pulls on the parietal pleura (since
the parietal pleura is attached to the thoracic body wall). You might think that
this would cause the two pleural membranes to pull apart, but that doesn't happen.
Instead, the serous fluid has enough surface tension to act like a glue
and hold the visceral pleura to the parietal pleura. This way, as the parietal
pleura is drawn out by the thoracic body wall, the visceral pleura moves with it. As
the visceral pleura expands, since it is stuck to the lung tissue, the lung is forced to
expand. This is how the pleurae lead to expansion of the lungs themselves.

Changes in thoracic pressure are prevented from affecting the
structure of the tiny alveoli

Since the alveoli are only air sacs with a single cell
thickness border, any change in air pressure could cause these sacs to buckle. And
even under no changes in pressure, if the luminal surface (the surface facing the lumen)
is covered with fluid, wouldn't that fluid try to stick to itself via surface tension and
cause the entire alveolus to buckle? Why doesn't this happen? Because the
apical edge of each alveolus is coated with a fluid called surfactant.
You see, most of the alveolar cells are regular, squamous epithelium cells.
But some of the cells are surfactant-secreting cells. So surfactant is released onto
the luminal surface of the alveolus.

Surface tension pulls fluids together, so much so that
it can hold membranes together, like the pleural membrane, or that it could cause an
alveolus to buckle. But surfactant is a fluid material that decreases surface
tension. When surfactant is on the surface of an alveolus, even during pressure
changes, the alveolus can hold its shape and not buckle.

Expiration

For the most part, we exhale passively. That means that we
don't have to expend any energy to exhale. You see, when our diaphragm has stopped
contracting for inspiration, it can now begin to relax. And the abdomen is under
higher pressure when the diaphragm is contracted (because the thorax expanded downward).
Therefore, the thoracic volume is restored by simple elastic recoil
of the diaphragm. Any other muscles that were active to cause inspiration are simply
shut off so that they relax for expiration.

We do have to be able to voluntarily exhale in order to speak,
whistle, play a wind instrument, blow up a balloon, etc. So, there are muscles that
contract for voluntary expiration. The major muscles involved are the internal
intercostal muscles and abdominal wall muscles that constrict the abdomen like
the internal and external obliques and the rectus abdominis
muscles. The internal intercostal muscles lower and constrict the rib cage to
decrease thoracic volume, while the abdominal wall muscles cause constriction of the
abdomen, forcing the abdomen to push against the diaphragm, shoving it back upward into
the thorax.